Polycrystalline CdS films were prepared by coating a slurry which consisted of CdS, $CdCl_2$, various amounts of $InCl_3$ and propylene glycole on glass substrate and by sintering in a nitrogen atmosphere, and their sintering behaviors, electrical properties and optical properties have been investigated. As the amount of $InCl_3$ increases, the enhancing effect of $CdCl_2$ on sintering decreases resulting in poor microstructure and a sharp decrease in optical transmittance. This phenomena have been explained in terms of reduction of $CdCl_2$ amount due to the occurrence of a chemical reaction which consumes $CdCl_2$ and grain growth inhibitation by the second phase precipitates. Electrical resistivity also increases sharply with increasing amount of $InCl_3$. The increase in resistivity is caused by the decrease in contact area between grains and not by the change in carrier concentration. The carrier concentration is almost independent of $InCl_3$ added due to the occurrence of chlorine doping of ~$10^{18}/cm^3$ and to the compensating effect of indium dopant. Theoretical calculation of the concentration of electrically active indium indicates that only an order of $10^{18}/cm^3$ of indium atoms, which is about the same as the carrier concentration due to the chlorine doping, is active in the sintered films. The electrical resistivity can be reduced by a heat treatment by which the contact area increases due the evaporation of the insulating layer between grains. Microstructure of CdS films, that contained $CdCl_2$ and $InCl_3$ before sintering, could be improved by sintering in a sealed boat.
